Originally Posted by
threemiles
There is always a danger in this thinking when not anticipating that loss of radar contact or comm failure may occur at some point. Exactly the same thinking was bringing the TU-154 and C-141 down.
Away from high dense airspaces and with the exception of the oceanic rush routes it is good practice to stick to semi circular rules over long thin traffic routes. No reason to not do so.
Things have evolved a little since introduction of radar in 1960. ATC radar Clearances today do not anticipate loss of radar . If that comes , emergency procedures, like 500 ft emergency separation, will try to save the day.
The 154/141 Namib collision you are refering to was outside radar coverage and the 154 was not talking to anyone , outside his original flight plan route , and probably ( we will never know) did not dare to change Altitude on his own initiative. There was no ATC intervention there.
In long thin procedurally controlled routes, semi circular makes a great deal of sense, I fully agree, but in our case here in Brazil, both aircraft were under radar control.
The reasons for (radar) ATC to deviate are many, the most common are intermediate level off ( in climb and descent), expediting other traffic thru a level, turbulence avoidance, complying with a local flight allocation scheme or an adjacent centre requirement to name a few.
Just think of 2 airways converging /crossing with 30 degr Westward , possibly even coming from 2 different units. You are of course going to use all levels at your disposal to solve the cruising conflicts , not only the even ones.